Modeling Long-Term Changes in Climate, Ice Sheets and Sea Level: Using the Paleo Record to Understand Possibilities for the Future
Bibliographic record
Abstract
The paleoclimate record highlights the susceptibility of ice sheets and sea level to increased global temperatures, even for global warming much less severe than that predicted for future climate. The critical role of climate feedbacks in regulating ice sheets over centuries and millennia highlights the need to use coupled ice-sheet/climate models in assessments of past and future sea level rise. However, coupled climate models are only beginning to include dynamic ice sheets, coupling mechanisms between ice sheets and climate, and the spatial resolution needed to properly simulate the coupled ice-sheet/climate system. During this project, we completed the development and testing of the infrastructure for coupled CESM2-CISM2 paleo-simulations of the Greenland ice sheet. The Community Earth System Model (CESM) is a fully coupled, global climate model that provides state-of-the-art computer simulations of the Earth's past, present, and future climate states. The newest version, CESM2, contributed simulations to the Coupled Model Intercomparison Project Phase 6 (CMIP6). The ice sheet component of CESM2 is the Community Ice Sheet Model Version 2.1 (CESM2.1), a parallel, scalable code with a suite of higher-order ice-flow solvers that are physically realistic for all parts of an ice sheet, including fast-flowing ice streams and outlet glaciers. The incorporation of a physically based “pseudo-plastic” basal sliding scheme gives realistic velocities over most of the Greenland ice sheet, while allowing basal conditions to evolve on multi-century time scales. Additional processes and feedbacks important for long, coupled, millennial-scale simulations – evolving topography, orbital acceleration, asynchronous coupling to dynamic vegetation – have been implemented. The preindustrial ice sheet/Earth system state is achieved via a new, efficient, interactive spin-up method and provides initial conditions for transient paleo-simulations with CESM2-CISM2, as well as the ISMIP6 coupled historical and future simulations. Taking advantage of these developments, we simulated the retreat and regrowth of the Greenland ice sheet during the Last Interglacial period from 127 to 119 thousand years ago – the first fully coupled, full-complexity global Earth system model and higher-order ice sheet model to successfully do so. The simulated evolution of the Greenland ice sheet is consistent with ice core and marine records for this time period. Our CESM2-CISM results suggest that the Greenland ice sheet contributed 4.2-meter sea level equivalent, with rates of sea level rise as high as one millimeter per year for several thousand years. The standalone Greenland ice sheet modeling indicates that there was melting across the whole ice sheet during the peak Last Interglacial warmth, with the central dome of the Greenland ice sheet shifted northward at this time, feeding both NEEM and the Summit ice cores. Glacial inception at the end of the Last Interglacial was also successfully simulated by CESM2-CISM2 – the results establishing a mechanistic link between glacial inception in North America and Scandinavia. The stability of the Greenland ice sheet under anthropogenic warming and its potential contribution to sea level rise over coming centuries and millennia is of vital societal importance. Past warm climate states are ideal proving grounds for models that are to be used for sea level projections. Thus, from a policy perspective, realistic simulation of previous warm periods lends confidence to assessments of future changes. The climate and Greenland ice sheet simulations of the Last Interglacial period benchmarked against paleo observations provides a robust validation of model performance in warm past climate states. To this end, our CESM2-CISM2 simulations of past warm states provide critical confirmation of CESM2-CISM2 and its use for assessing future ice-sheet/climate evolution and sea level rise.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.002 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.001 |
| Bibliometrics | 0.000 | 0.001 |
| Science and technology studies | 0.001 | 0.001 |
| Scholarly communication | 0.001 | 0.002 |
| Open science | 0.001 | 0.001 |
| Research integrity | 0.001 | 0.001 |
| Insufficient payload (model declined to judge) | 0.002 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".